Due to the widespread of renewable energies, construction of mega solar systems are accelerated. In a case of a mega solar system of 1 MW volume, approximately 5000 pieces of panels are required. Consequently, a vast extent of land is covered with panels. Solar power generation systems are DC power supply, and at present, generally, they are low voltage lower than 750V, however, in future, their voltage might reach 1000 V. Further, due to degradation of cable, salt damage, etc., inferior electrification or contact resistance might increase, therefore, the probability of DC arc failure (series arc, parallel arc) might increase. In a case where the above-mentioned failure occurs, it is difficult to specify fault points in huge systems such as a mega solar system. Further, a current, which flows when a series arc or a parallel arc occurs, will not reach a current which trips a breaker. Consequently, it is necessary to establish protection method of solar power generation systems so as to prevent from continuing an arc in a case where an arc occurs.
Arc fault detection and protection of solar power generation system is disclosed in Patent Documents 1 to 3, for example. According to the Patent Document 1, a difference between power which is supplied to a load and power which is generated by photovoltaic panels is obtained, and in a case where a difference of the power is larger than a threshold value, an alarm condition is set. Alternatively, noise voltage of a load and noise voltage of photovoltaic panels are measured and a difference of the voltage is obtained, and in a case where a difference of the noise voltages is larger than a threshold value, an alarm condition is set. In a case where a difference between power which is generated by photovoltaic panels and power of a load is larger than a threshold value, an alarm condition is set. However, in a large-scale plant of a solar power generation system such as a mega solar system, power loss due to loss of back flow preventing diode or distribution cable and deterioration of a cable occurs. Consequently, it is difficult to judge occurrence of an arc only by power difference.
According to the Patent Document 2, an arc detector is provided at an end part of a power line and on a retrace line, and an overcurrent protection, an arc failure protection and a reverse-current protection can be performed. However, an arc detection is performed only by parallel arc judgement with a reverse-current detector.
According to the Patent Document 3, arc detection is performed based on variation of voltage-current, however, a case in which a constant voltage source is used is supposed. A voltage-current characteristic of a solar power generation system is different from that of a constant voltage source, therefore, judgement method is different. Further, judgement of an arc occurrence point or an arc mode is not performed, therefore, an arc failure point cannot be specified. Further, a case of plural parallel arcs is not supposed, and selective breaking technique is not included.